As portable electronic devices, such as cellular phones, computer tablets, and the like, become more prevalent, demand for compact, lightweight filters used in these electronic devices may also be rising. In many electronic devices, electrical resonators may be used as filters. For example, in many wireless communications devices, radio frequency (rf) and microwave frequency resonators may be used as filters to improve reception and transmission of signals. Filters typically include inductors and capacitors, and more recently resonators.
Film bulk acoustic resonators (FBARs) may be used as compact, lightweight filters in these electronic devices. FBARs may be inexpensively mass-produced and may be implemented in a micro-structure. A FBAR may have a multi-layer resonance section in which a bottom electrode, a piezoelectric layer, and a top electrode are sequentially provided in this order. The FBAR may use a piezoelectric phenomenon, by which when electric energy is applied to the top and bottom electrodes, piezoelectric effect may be produced and resonance results. In such a FBAR, separation between the substrate and the multi-layer resonance section may be needed, so that acoustic waves generated from the piezoelectric layer are not affected by the substrate.
Recent advanced fabrication modalities have led to face-mounted resonators, which are commonly known as solidly-mounted resonator (SMR). Acoustic stack plates in SMRs have been used only for their mechanical properties as impedance transformers, but stacks with a piezoelectric layer provide an additional degree of freedom, because the electrical boundary condition on the piezoelectric layer may be altered.
Currently, aluminum nitride (AlN) films that may be used for acoustic resonators are polycrystalline piezoelectric films deposited by reactive sputtering over a limited selection of metals and dielectrics that are suitable for the growth of reasonable quality films. For example, Film Bulk Acoustic Resonators (FBARs) are generally fabricated by sputtering the piezoelectric film over a semiconductor wafer. Solid Mounted Resonators (SMRs) may be fabricated by sputtering the piezoelectric film over a wafer with Distributed Bragg Reflectors (DBR). Sputtered piezoelectric films are polycrystalline and their quality, uniformity and reproducibility depend on growth conditions, substrate conditions, and layer thickness. Thus, it may be difficult to form resonators having high quality, uniformity and reproducibility.
Therefore, it would be desirable to provide a device and method that overcomes all of the above.